is the simplest form you can get. Of course you can insert
explicit components using Mathematica and do the multiplication
of spinors and Dirac matrices, but for that you don't really need
FeynCalc. On the other hand, if you want to compute matrix element
squared, then you cold do say

h2 = FermionSpinSum[h ComplexConjugate[h]] // Tr

and then replace scalar products with some explicit values that are
given by the kinematics you choose,like